People act as if IM1 made 1 billion dollars, it only made 130M more than Thor. Loki by himself is arguably just as popular as Stark, both Thor and Loki are alot more popular after The Avengers.

I believe that, specially if it's great, it will double his international gross, so it will make at least 500M there.
Considering it will do more more than 200M in US, I see 700M as the floor for this movie, really.